Time, speed, distance

Story hook

A father drives 60 km to drop his son at school each morning, covering it in 1 hour. The son says: "Papa, what's your average speed?"

"60 km/h", says the father.

On the way back home, traffic is bad — he covers the same 60 km in 2 hours.

"What was the average speed for the round trip?", the son asks.

Most people answer "50 km/h" (average of 60 and 30). It's wrong. The correct answer is 40 km/h (harmonic mean).

CSAT exploits this specific intuition trap in 1-2 questions every year. Plus 4-5 questions on trains crossing platforms, boats in streams, two-vehicle approach problems, and circular tracks.

After this unit, every Time-Speed-Distance question (TSD) on CSAT will be a 30-second pattern match to one of 6 templates.

Why this matters for UPSC

For CSAT (Paper II, qualifying 33%):

• 6-10 questions per paper are TSD. Plus 2-3 from trains-and-boats subgenre.
• TSD intuition shows up in GS-III economy (transport costs), GS-I geography (monsoon arrival timing, cyclone speed), GS-III science (mechanics basics).
• Daily life: Estimating commute time, train arrivals, flight durations.

This is the third highest-yielding topic in CSAT (after percentages and number system). 2-3 hours of practice = ~12% of CSAT score locked in.